1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, and more particularly to an image forming apparatus equipped with imaged density control means for determining an appropriate image forming condition at the image formation according to the measurement of the density of the original document.
2. Description of the Prior Art
Conventionally there are known following two methods for controlling the image density in this manner:
(1) a method of measuring and storing the original density, and suitably regulating image forming conditions such as the intensity of the exposure lamp or the developing bias according to thus stored original density at the image formation; and
(2) a method of successive comparison in which the image forming conditions are feedback controlled according to the original density read during the image formation.
In the method (1) in which a pre-scanning for measuring the original density is conducted prior to the image formation, the continuous copying operation is conducted, regardless of the number of copies desired, according to an automatic exposure (AE) value corresponding to the original image density determined by the pre-scanning. However in case an elevated number of copies are formed in a continuous copying operation, the image forming conditions often vary between the start and the end of the continuous copying operation. Consequently the image density on the obtained copies may vary even if the AE value is maintained constant.
Also said pre-scanning should preferably be conducted over the entire area of the original, it is often conducted only over a part of the original in order to avoid loss in the copying speed. An exact measurement of the original density cannot be expected in such case if the original contains for example a solid black area in such measured part.
On the other hand, in the latter method in which is the original image density is measured in succession simultaneously with the image formation, a precise feedback control according to the original image density becomes difficult due to a delay in the feedback for example in case the original contains black and white areas in repetition.
Also in the conventional methods, the measurement of the AE value and the determination of the control values for operable conditions such as the light intensity of the exposure lamp, and the potential of the developing bias etc. have to rely on a time-consuming logic processing according to a determined formula.